1. Field of the Invention
The present invention relates to a fixing device that fixes a toner image onto a recording medium while conveying the recording medium by causing the recording medium to pass through a nip portion formed by a heating member and a pressing member and an image forming apparatus such as a copying machine, a printer, a facsimile, or a multifunction peripheral (MFP).
2. Description of the Related Art
In the image forming apparatus of an electrophotographic system described above, a fixing device is provided to fix a toner image onto the recording medium while the recording medium is conveyed by passing through a nip between the heating member and the pressing member. This type of fixing device has a configuration in which the pressing roller serving as a pressing member is brought into pressure contact with a fixing unit (a roller or a belt) having a heat source therein, a transfer sheet with an unfixed toner image thereon passes through a pressing portion between the roller or the belt and the pressing roller, and thus the toner is fixed onto the transfer sheet. The surface of the fixing member is coated with a silicone rubber or a fluorine coating material, for example, so as to prevent the toner from adhering to the surface thereof. However, depending on a condition such as a usage environment or a type of the transfer sheet, some amount of toner may leave the transfer sheet and adhere to the fixing roller, being called an offset phenomenon.
It is known that the offset toner remains on the fixing roller or the pressing roller that may be brought into contact with the fixing roller and may be reversely transferred onto the transfer sheet, thereby causing an image to be contaminated. In order to prevent such a problem from occurring, the fixing roller or the pressing roller may be in contact, as a cleaning means, with a swabbing roller or a take-up web that contains oil.
The minutely offset toner may move from the fixing roller or the pressing roller to the swabbing roller due to a difference in the mould releases, or may adhere to the surface of the web because the web is in contact with the roller by terminating the rotation of the roller. The offset toner adhered to the web is removed by being taken up and thus a new surface thereof is always brought into contact with the fixing roller or the pressing roller.
In general, the web has a function, as disclosed in Japanese Patent Application Laid-open No. 2008-129279, such that cleaning performance can be improved for large impregnation of oil by adsorbing the toner and the cleaning performance may be degraded for small impregnation of oil due to the improvement of the oil swabbing performance. In Japanese Patent Application Laid-open No. 2008-129279, the web has two contact portions where the web has different impregnation of oil so as to maintain the cleaning performance and the oil swabbing performance. However, there is concern that abnormality such as an image stripe may occur if the swabbing amount of oil supplied from the web is increased.
Conversely, if the impregnation amount of oil in the web is simply reduced, absorption (collection) of the offset toner to the web becomes insufficient. Accordingly, part of the offset toner evades from the web, causing reverse transfer of the offset toner to the transfer sheet, or the part of the offset toner passes through the nip portion again and is stopped by the web again in a state with a reduced viscosity, resulting in the offset toner being adhered to the portion. The adhered portion may cause damage on the fixing roller or the pressing roller by being in contact therewith.
In addition, the toner remaining on the web is not in a state of being sufficiently heated immediately after the activation from the cooled state or the returning from the off mode. Furthermore, if the toner has not yet been heated enough so as to enter a viscous state, the toner cannot be absorbed and fixed to the surface of the web, so that the cleaning performance is insufficient. Even in a case where a large amount of a releasing agent is contained, the toner is in a floating state on the fixing roller or the pressing roller due to the oil supplied from the web. When a paper sheet passes in this state, the offset toner floating on the fixing roller or the pressing roller may move to the transfer sheet to cause toner contamination. This phenomenon occurs significantly in an apparatus that has a short activation period.
Recently, the fixing device is configured such that a heat source (heater) is disconnected from a power supply in a standby mode in order to reduce a value of the typical electricity consumption (TEC) or to save energy. Furthermore, the heat source is supplied with power when the image formation starts, and the fixing roller and the like are heated up to the temperature sufficiently high for fixing the toner. For this reason, the fixing roller or the heating roller with which a belt is suspended is formed to be thinner than 1 mm or less, so that thermal responsiveness is improved and the activation time taken for fixing the toner is shortened. Therefore, usability by a user and energy saving are achieved. With this configuration, because the thermal capacity of the fixing roller or the heating roller is small, the fixing roller or the heating roller is easily affected by the thermal migration to the transfer sheet or to a contact member of the fixing roller or the fixing belt, by the flow of air, or by the like so that the temperature distribution may vary largely depending on a position of each element.
If the temperature distribution in the fixing roller is not uniform, the offset may occur due to the disagreement in the fixing conditions or the lifetime of the roller may be shortened due to the thermal deterioration. A particularly serious problem is a reverse transfer in which the toner lump deposited on the cleaning roller or the web melts again and is transferred onto the transfer sheet. The problem is serious when a size of a sheet is smaller than the maximum size of the sheet that can be passed (because a sheet with a small size is in contact with the fixing roller in a small area, from which heat is robbed by the sheet, resulting in a decrease in the temperature only in the sheet passing portion without causing a decrease in the temperature in a sheet non-passing portion, such as the fixing roller or the fixing belt. Therefore, because a temperature detecting unit that is provided to a corresponding portion instructs a heater to turn on, the temperature of the sheet non-passing portion increases unnecessarily and thus the toner adhered to the cleaning roller corresponding to the portion also melts, causing the toner to be reversely transferred).
In other words, when an amount of toner that adheres to the cleaning roller or the web is increased and heat is applied to the adhered toner due to continuous passing of the sheets, the adhered toner melts and the viscosity thereof is lowered, the offset toner is decoupled from the adhered toner on the surface of the cleaning roller, and the collection performance (adhesiveness≈cleaning performance) may be degraded. As a result, the offset toner adhered to the cleaning roller or the web is reversely transferred and the so-called melting out of the toner may occur. The offset toner occurs more easily as the image area becomes larger, and the reverse transfer may be caused by a rise in the temperature of the cleaning roller or the web due to the continuous passing of the sheets.
As a method for preventing the melting out, the cleaning roller or the web may be provided on the pressing roller side where heat is not easily increased. Furthermore, in Japanese Patent Application Laid-open No. 2005-266746, a cross-linking agent is coated on the cleaning roller to prevent the melting out from occurring. In this method, the cross-linking agent is applied onto the surface of the cleaning roller in advance so as to cause the cross-linking agent to react with the toner (cross-linking reaction), thereby increasing the viscosity of the toner and preventing the melting out from occurring. The cross-linking agent is dispersed in the adhered toner to perform the cross-linking reaction. Therefore, the toner that is collected to the cleaning roller is in a highly viscous state, and this effectively prevents the melting out from occurring. However, the method is disadvantageous in terms of the toner contamination on the transfer sheet immediately after the activation.
In a case where the cleaning roller or the web is made to be in contact with the pressing roller, the oil impregnated in the cleaning roller or the web is supplied to the fixing roller only in an inter-sheet interval during the passage of the sheets. Therefore, an image stripe (offset) is easily generated due to oil-caused gloss on the image.
For this reason, it is desirable to reduce the impregnation amount of oil to the web so as to suppress the occurrence of the image stripe caused by the oil. At the same time, it is required to provide an apparatus that maintains the cleaning performance by suppressing the occurrence of an abnormal image caused by the offset toner.
Therefore, there is a need for solving the above described problem and for providing an image fixing apparatus and an image forming apparatus that can reduce a problem that a recording medium may be contaminated by evading a cleaning unit of the offset toner.